Uniform magnetic field designs are often based on the Helmholtz coil consisting of two circular coils of radius a each carrying a current I and spaced a distance d apart. If d=a, this results in a magnetic field that that has zero first and second spatial derivatives at the midpoint of the line at z=a/2 joining the centers of the two coils, and thus is highly uniform in the central region between the coils.
At all other points, the magnetic field is spatially varying so that a single pair of Helmholtz coils alone is unsuitable in conventional MRI machines that require extremely uniform magnetic fields. However, an advantage of the Helmholtz coil arrangement is that the central region is easily accessible from the outside.
The magnetic field is approximately uniform in the central region of long thin solenoid coils although the magnetic field is non-uniform in the fringing field near the ends and outside the solenoidal coil. This shape is used in conventional MRI machines to accommodate a full body lying flat.